In electrophotographic image formation, an electrostatic latent image is formed on a photoconductive image bearer, a charged toner is attached to the electrostatic latent image to form a visual toner image, the toner image is transferred onto a recording medium such as a paper and fixed thereon. Recently, electrophotographic copiers and printers have rapidly developed from monochrome to full-color, and the full-color market is expanding.
Electrophotographic full-color image formation typically uses three primary colors yellow, magenta and cyan toners or four color toners including a black toner, adjusts contrasting density of each color toner image and overlaps each of the color toner images to reproduce all colors. However, occasionally a following image takes over a history of the last image (ghost phenomenon), and when a toner image varies in density, the resultant image varies in color toner.
Conventionally, a one-component developing method, a two-component developing method and a hybrid developing methods are used, and each of the methods is thought to have a different occurrence mechanism of the ghost phenomenon from each other.
Namely, in the one-component developing methods, when a residual toner unconsumed in the developing process returns in the image developer, the toner is not completely scraped off from the feed roller and remains on the surface of the developing roller to be used in the following development. The residual toner is mechanically scraped off at a toner feeding part and the toner having a large particle diameter is relatively easy to scrape off and the toner having a small particle diameter gathers at a developing part, and which causes a specific charge variation, resulting in the ghost phenomenon.
The hybrid developing method forms a magnetic brush formed of a non-magnetic toner and a magnetic carrier on the outer circumference of a magnetic roller, feeds only the non-magnetic toner to a toner bearer from the magnetic roller to form a uniform toner layer, and applies the toner of the toner layer to an electrostatic latent image on an image bearer. A specific amount of the toner is constantly fed to the toner bearer and the toner amount thereon is varied due to the last image, resulting in the ghost phenomenon.
Namely, when the last image consumes less toner, the toner remains more on the toner bearer, and the toner thereon further increases after the toner is fed and the resultant image has higher density. Meanwhile, after an image consuming more toner is produced, the toner remaining on the toner bearer decreases. The toner amount on the toner bearer is relatively less after the toner is fed and the resultant image has lower density.
As mentioned above, the ghost phenomenon in the hybrid developing method is caused by the toner amount variation on the toner bearer when a following image is produced according to the history of the last image because it is difficult to uniform the amount of the decreased toner after used for development and the amount of the undeveloped toner remaining on the toner bearer when the toner is transferred onto the toner bearer from the magnetic brush.
In order to solve these problems, Japanese Patent No. 3356948, and Japanese published unexamined applications Nos. 2005-157002 and 11-231652 disclose scraping off the toner remaining on the toner bearer therefrom with a scraper or a toner collection roller after developed and before fed again. Japanese published unexamined application No. 7-72733 discloses a method of collecting the toner remaining on the toner bearer on a magnetic roller by potential difference between copyings or papers to stabilize the toner amount on the toner bearer. Further, in order to solve the problem of history development using the magnetic brush, Japanese published unexamined application No. 7-128983 discloses widening a half width area of a magnetic flux density of the magnetic roll to collect and feed the toner on the toner bearer. Japanese published unexamined application No. 6-92813 discloses a method of using a non-spherical carrier to increase the surface area thereof and increasing a ratio of the carriers contacting each other to charge the carrier even at the end of the magnetic brush, narrowing a substantial gap between the developer bearer and the toner bearer to increase the toner amount fed to the toner bearer at a time, and feeding the toner until the toner bearer is saturated with the toner to maintain a specific amount of the toner on the toner bearer and prevent an influence of the last image history.
Even the two-component developing method has the ghost phenomenon. Poor separation of the developer is thought to cause the ghost phenomenon.
The two-component developing method has an odd number of magnets in the developer hearer and a pair of magnets having the same polarity below the rotational axis of the developing sleeve to form a separation area where a magnetic force is almost zero. The developer naturally falls there by gravity to separate from the developer bearer.
However, the carrier has a counter charge when the toner is consumed in the last image, and an image force generates between the carrier and the developer bearer and the developer does not separate at the separation area. The toner is consumed and the developer having a lowered toner concentration is fed to the developing area again, resulting in production of images having low image density. Namely, images having normal image density are produced for one cycle of the sleeve, but the image density lowers since the second cycle, resulting in the ghost phenomenon.
In order to solve these problems, Japanese published unexamined application No. 11-65247 discloses a configuration of locating a scoop roll having a magnet inside at the separation area above the developer bearer to separate the developer after developed by the magnetic force. The separated developer is further scooped up by another scoop roll, and fed to a developer stirring chamber where the toner concentration is adjusted again and the toner is charged.
However, even when the toner concentration is adjusted again and the toner is charged, the ghost phenomenon occasionally occurs. The mechanism of this ghost phenomenon is not clarified, but it is thought the toner adhered to the developer bearer according to the last image history and the toner amount developing the following image varies according to a potential of the toner having adhered to the developer bearer.
Specifically, the toner adheres to the developer bearer because a bias is applied in a direction of the developer bearer in a non-image forming area and the toner in the development area is developed on the developer bearer. Having a potential, the toner developed on the toner bearer increases the development potential on the part where the toner is developed, resulting in increase of the toner amount for development.
Meanwhile, a carrier capable of stably forming images against potential or environmental variation is studied.
Japanese Patent No. 3755289 discloses a carrier including a specific metallic atoms such as iron, an alkali metal or an alkali earth metal in its silicone-resin coated layer to induce a charge accumulated on the surface inside to prevent accumulation of the charge on the surface. However, the metallic atoms independently present in the silicone-resin coated layer do not sufficiently induce the charge.
Japanese published unexamined applications Nos. 2010-256759 and 2009-109814 and Japanese Patent No. 3298034 disclose a resin-coated carrier exposing its core material on the surface at a specific ratio. However, a size of one of the exposed parts of the core material is not disclosed. When an area of the exposed part of the core material is large, the carrier is vulnerable to moisture and a charge is easy to leak.
Japanese Patent No. 3904205 discloses an average area ratio of one of the exposed parts of the core material not greater than 0.03%. However, the resin-coated carrier exposing its core material on the surface has thin layer thickness around the exposed part and the exposed part is vulnerable to stress, resulting in deterioration of durability.
Japanese published unexamined applications Nos. 2009-180820 and 2008-203624 disclose a resin-coated carrier, the resin layer of which includes an electroconductive particulate material to control resistivity of the resin layer. However, the resin layer does not have sufficient durability.
Because of these reasons, a need exist for a carrier having good durability, consuming a stable amount of a toner for development without influence of the toner consumption history of the last image, and producing uniform images having good color reproducibility for long periods.